The present invention relates to an optic superconducting circuit element that is useful in the field of superconducting micro-optoelectronics, which provided with both superconducting Josephson electromagnetic wave oscillating (generating and transmitting) and receiving junctions is capable of acting to oscillate (transmit) and to receive an electromagnetic wave having a frequency in a THz frequency band.
So far, ac Josephson (alternating current) effect detecting device has been known as a superconducting, electromagnetic wave detecting apparatus.
As far as the detection of an electromagnetic wave is concerned, B. D. Josephson has made a proposition (see Physics Letters, Vol. 1, 251-253, 1962) that biasing a superconducting junction at a finite voltage draws an extra-high frequency alternating current through the superconducting junction, but the irradiation of an electromagnetic wave on the superconducting junction from its outside causes these electromagnetic waves to be mixed together and a current step to appear at a voltage corresponding to a beat caused by two slightly different frequencies of those waves, from which current step the input electromagnetic wave can be detected. The Josephson""s proposition, which relates only to the principles of detection, makes no mention of any detailed method of such detection, however.
S. Shapiro et al conducted a test designed to verify the principles of detection proposed by Josephson through the use of an Al/AlOx/Sn junction and first observed such a current step, which was induced by the irradiation with a single-frequency microwave. They made no mention, however, of any high sensitivity technique that may be used to detect a spectrum over a terahertz (THz) frequency range. See Review of Modern Physics, Vol. 36, 223-225, 1964.
In an attempt to detect electromagnetic waves over a wide infrared frequency range by using a Josephson junction, Grimes et al have biased the Josephson junction at a fixed current, irradiated the junction with such an electromagnetic wave turned on and off alternately, and measured a difference in potential across the junction as an electromagnetic wave response. See Journal of Applied Physics, Vol. 39, 3905-3912, 1968.
In this method of measurement, not only does the use for the Josephson detector of a low temperature superconductor that is small in its energy gap make it difficult to measure an electromagnetic wave in a THZ frequency range, but also its oscillating (generating and transmitting) source located remote from the detector makes it hard to detect the oscillating (generating and transmitting) source of the electromagnetic wave if weak in its intensity.
Kanter, Vernon et al have used a point contact Josephson junction made of Nb to detect broadband electromagnetic waves in the neighborhood of 90 GHz. See Journal of Applied Physics, Vol. 43, 3174-3183, 1972. In this method of detection, too, the source of oscillation (transmission) of the electromagnetic waves and their detector not of an on-chip construction but located remote from each other and the use of a low temperature Nb superconductor to form the Josephson junction for detection makes it hard to detect electromagnetic waves over a broad THz frequency band.
Further, Divin et al have conducted a theoretical analysis and basic experimentation in order to apply the Josephson AC effect to the spectrum detection of incoherent broadband electromagnetic waves (see IEEE Transactions on Magnetics, Vol. MAG-19, 613-615, 1983) and, on obtaining apposite results confirming the agreement of the theory with the experiments, built up of the foundation of the detection of broadband electromagnetic waves by the use of the Josephson junction.
In the experimentation, it was successful to detect electromagnetic waves in the neighborhood of 600 GHz by using the junction that was of a Nb point contact type. Here again, the use of Nb as a low temperature superconductor that is smaller in energy gap than a high temperature superconductor makes it difficult to detect electromagnetic waves largely broadened over a THz frequency band. Yet further, no mention is made at all in the report of a concept to improve the sensitivity of detection by adopting an on-chip construction.
Thus, while there have been precedents in which the principles of detecting and the method of measuring broadband electromagnetic waves by the use of a Josephson junction are applied to an electromagnetic wave detector, there has not yet been made extant any optic superconducting circuit element operable to oscillate (transmit) and receive an electromagnetic wave over an extended frequency band ranging from microwave to THz frequency bands.
With the foregoing taken into account, it is accordingly an object of the present invention to provide an optic superconducting circuit element that is highly sensitive and operable to oscillate (transmit) and receive broadband electromagnetic wave of frequencies ranging from a microwave to a THz ranges.
In order to achieve the object mentioned above, there is provided in accordance with the present invention an optic superconducting circuit element that comprises a superconducting electromagnetic wave oscillating (generating and transmitting) source for oscillating (generating and transmitting) an electromagnetic wave and a superconducting Josephson junction device for receiving the electromagnetic wave transmitted from the said superconducting electromagnetic wave oscillating (generating and transmitting) source, and wherein the said superconducting electromagnetic wave oscillating (generating and transmitting) source and the said superconducting Josephson junction device are mounted on a single chip.
Preferably, the said superconducting electromagnetic wave oscillating (generating and transmitting) source and the said superconducting Josephson junction device are so mounted as spaced from each other at a distance not greater than 1 mm.
Preferably, the said superconducting Josephson junction device has a (Josephson) junction adapted to receive the said electromagnetic wave transmitted from the said superconducting electromagnetic wave oscillating (generating and transmitting) source, and the circuit element further comprises a means for detecting the said electromagnetic wave in response to ac Josephson (alternating current) effect brought about at the said junction by the said electromagnetic wave incident thereon.
The optic superconducting circuit element according to the present invention preferably further includes a means for biasing the said superconducting Josephson junction device with a fixed electric current and shifting the point of such biasing successively in response to a change occurring according to the presence or absence of the electromagnetic wave, thereby providing continuous spectral information thereof.
The said electromagnetic wave may have frequencies in a frequency band ranging from microwave to THz frequency bands.
Also, in the optic superconducting circuit element according to the present invention, each of the said super-conducting electromagnetic wave oscillating (generating and transmitting) source and the said superconducting Josephson junction device is made preferably of a superconductor that is large in superconducting energy gap to extend the detectable frequency band of the electromagnetic wave to cover a THz frequency band. The said superconductor is advantageously a high temperature oxide superconductor.
Further, in the optic superconducting circuit element according to the present invention, each of the said super-conducting electromagnetic wave oscillating (generating and transmitting) source and the said superconducting Josephson junction device is formed of a thin film, or a thin film, single crystal film superconductor.
In an optic superconducting circuit element constructed as mentioned above, an electromagnetic wave made incident onto the Josephson junction biased at a fixed voltage is mixed with a high frequency electric current brought about by ac Josephson effect to cause it to appear as a change in its DC component. If the input power signal is sufficiently small, its change has a zero-order change thereof primarily contributing thereto.
Detection by ac Josephson effect satisfies the conditions prescribed by the relationship between the Josephson voltage and the frequency. Accordingly, if the input electromagnetic wave has a narrow frequency band, sharp responses appear at voltages corresponding to its frequencies. If, however, the input electromagnetic wave is weak and has a broad frequency band, then there does it appear as a continuous spectrum over a wide voltage range.
The present invention in which a superconducting electromagnetic wave oscillating (generating and transmitting) source is provided on a same chip on which is provided a superconducting Josephson junction device as a detector for an electromagnetic wave from the source and moreover in which the distance between the oscillating (transmitting) source and the electromagnetic wave detector can be set, e.g., within the order of 1 mm, makes it possible to receive according to the Josephson junction detection principle a weak or feeble power signal in a broad frequency band and with high sensitivity.
The frequencies detectable here are determined by the energy gap of a high temperature superconductor used for the detector and in principle extend even to the order of 10 THz.
The present invention, thus by joining a high sensitivity superconducting electromagnetic wave detector and an oscillating (a transmitting) source together in close proximity, makes it possible to realize an optic superconducting circuit element capable of performing a high sensitivity, broad band signal oscillation (transmission) and reception that has never been attainable heretofore.